what is ipad update error 4000

This support page makes reference to 4000-series errors:

If you see error 9, 4005, 4013 or 4014 when you restore your iOS device – Apple Support

Whilst Error #4000 is not explicitly listed in this support page, Apple Support are known to recommend that you follow these steps when troubleshooting this error - prior to seeking further assistance from Apple Support.

Apple Support can be contacted directly using the Contact Support link at top-right of this page - or by using the excellent Apple Support App. If the App is not already installed, it can be downloaded from the App Store:

https://apps.apple.com/gb/app/apple-support/id1130498044

Here are a few suggestions:

Try Rebooting both your iPad and your computer.

Make sure your iPad is fully charged.

Try another cable, or another USB Port.

If you are using PC and therefore iTunes, make sure iTunes is updated to latest version.